1. Field of the Invention
The present invention generally relates to implantable medical devices, such as cardiac pacemakers and implantable cardioverter/defibrillators, and in particular to a method, a medical device, a computer program product and a computer readable medium for determining the posture of a patient.
2. Description of the Prior Art
A severe problem associated with measurement of, inter glia, the blood pressure of the chambers, contractility, endocardial acceleration, blood flow, coronary blood flow, the sinus rate, the electrical bio-impedance, such as the thoracic impedance and the cardiogenic impedance is that the accurateness and reliability, and, hence the repeatability, of the obtained signals are greatly affected by factors like the body position of the patient, patient activity levels, heart rate frequency, etc. In particular, it has been found that the body position of the patient is of major importance with regard to the blood pressure of the chambers, contractility, endocardial acceleration, blood flow, coronary blood flow, the sinus rate, the electrical bio-impedance, such as the thoracic impedance and the cardiogenic impedance, etc. Repeatable measurements of such parameters are of a great value for identifying changes of many different conditions in the body of a patient. For example, for many algorithms it is important to know the position or posture of the patient to be able to compare obtained data to data recorded previously, for example at pulmonary edema monitoring, measurements of electrical bio-impedance (e.g. cardiac impedance) in order to calculate trends on how a patient's disease progresses or on how a certain condition progresses in a accurate and reliable way. Electrical bio-impedance signals have, for example, been found to constitute an effective measure for identifying changes of many different conditions in the body of a patient, such as incipient pulmonary edema and the progression of pulmonary edema due to CHF, i.e. the accumulation of fluids in the lung-region associated with pulmonary edema affects the thoracic impedance, or more specifically the DC impedance level, since the resistivity of the lung changes in accordance with a change of the ratio of fluid to air.
Clinical and pre-clinical measurements have, in fact, shown that the electrical bio-impedance is dependent to a significant extent also regarding different positions even when the patient is lying down, for example, whether the patient is lying on a side or is lying on the back. Regarding impedance measurements, a major reason for this is that the measurement depends on the measurement vector, i.e. the vector between the nodes that the current is applied between and the vector the voltage is measured between. When the body shifts position, these vectors will change since the gravity will influence, for example, tissue between the nodes and how it moves. This significant posture dependence applies both when looking at the morphology of the impedance signal in general and the DC-levels of the impedance signal. Pre-clinical trials on sheep have shown that the DC-level of the impedance may vary as much as 25-40% depending on the subject's posture: This is true for both the thoracic impedance, for example, measured over the lungs (for example between a case of an implantable device and an electrode placed within the heart) and the cardiogenic impedance, for example the DC impedance measured within the heart (for example RV-RA or RV-LV).
Accordingly, there is a need for a method and medical device that are capable of determining the posture of a patient in an accurate and reliable way.